1. Field of the Invention
The present invention relates to a power supply apparatus for laser which supplies an electric power to an excitation lamp for use in laser oscillation.
2. Description of the Related Art
Solid-state laser equipment using YAG laser etc., has such a configuration that an optical energy obtained when an excitation lamp is turned on is applied to a YAG rod or other laser medium, to activate laser oscillation.
FIG. 10 shows a circuit configuration of a conventional power supply apparatus for laser used in this type of solid-state laser equipment.
In this power supply apparatus for laser, output terminals [OUTa, OUTb] are connected respectively to two electrodes of an excitation lamp (not shown) of a laser oscillation unit.
A three-phase rectifying circuit 200 on the input side rectifies a commercial-frequency three-phase AC voltage supplied from a three-phase AC power supply terminals [U, V, W] into a DC voltage. From the three-phase rectifying circuit 200, a charging DC current i.sub.c flows via a magnetic contactor 202 and a smoothing coil 204 into a capacitor 206 to charge it to a predetermined voltage.
Between the capacitor 206 and the output terminals [OUTa, OUTb] , a switching element 208 is connected in series. When the switching element 208 is closed, the capacitor 206 is discharged, so that the discharging current iL flows into the excitation lamp via the switching element 208, an inductance coil 210, an output capacitor 212, and a reverse-current preventing diode 214. The lamp current iL turns the excitation lamp on.
When the switching element 208 is opened, the capacitor 206 is interrupted in its discharge, but electromagnetic energy and electric charge energy saved on the inductance coil 210 and the output capacitor 212 respectively are discharged instead, thus permitting the lamp current iL to keep on flowing.
The discharging switching element 208 is closed and opened by, for example, a 50 kHz high-frequency switching signal CS supplied from a control unit 220. With this, the lamp current iL can flow continuously and the excitation lamp can be on uninterruptedly, to obtain a continuously oscillated laser beam from the laser oscillation unit. The rated voltage applied by such continuous oscillation onto the excitation lamp is, for example, of the order of 150V.
Between the reverse-current preventing diode 214 and the output terminal OUTa, a current sensor 216 is provided to detect the lamp current i.sub.L. Based on an output signal fed out from the current sensor 216, a lamp current detection signal S.sub.iL is obtained which represents the magnitude of the lamp current, for example its effective current value. The lamp current detection current S.sub.iL is then sent to a control unit 220.
The control unit 220, based on the lamp current detection signal S.sub.iL from the current detection circuit 218, controls the switching action of the switching element 208 so as to provide a matching between the lamp current i.sub.L and a preselected current value. Moreover, if the lamp current i.sub.L flows excessively due to the breakage, damage, etc. of the switching element 208, the control unit 220 breaks the magnetic contactor 202. It is noted that a resistor 203 connected in parallel with the magnetic contactor 202 is a current-limiting resistor.
This type of power supply apparatus for laser comprises, as well as the main power supply unit to provide laser oscillation power to the excitation lamp as mentioned above, a trigger circuit (not shown) and a booster circuit for initiating the lighting of the excitation lamp.
In the illustrated conventional power supply apparatus for laser, one-phase AC voltage e (220V) of the three-phase AC power supply voltage is supplied to the primary coil of a step-up transformer 224, thus causing a step-up voltage, e.g. 1000V obtained across the secondary coil of the step-up transformer 224 to be applied to the booster circuit 226. Then, the booster circuit 226 rectifies the AC voltage from the transformer 224 through diodes d.sub.1 and d.sub.2 and charges in series the rectified voltage on capacitors c.sub.1 and c.sub.2 up to, which is stepped up to, e.g. 2,500V, with the boosted high voltage being then applied via a resistor 228 and a reverse-current preventing diode 230 to the excitation lamp as a boost voltage E.sub.f.
Before turning the excitation lamp on, the control unit 220 first activates the main power supply and the booster circuit 226. That is, for the main power supply unit, the control unit 220 closes the magnetic contactor 202 to supply the switching control signal CS to the switching element 208. For the booster circuit 226, the control unit 220 closes a switch 222 provided on the primary circuit of the step-up transformer 224.
Thus, with the main power supply and the booster circuit 226 being on standby, the control unit 220 operates the trigger circuit (not shown). With this operation of the trigger circuit, a gas within the excitation lamp is broken down, to rapidly lower the lamp impedance. When, on top of that, a boost voltage Ef of approximately 2500V is applied onto the excitation lamp from the booster circuit, the excitation lamp impedance drops further, causing a current to flow in the excitation lamp. Thereafter, even a low lamp voltage of the order of 150V from the main power supply is enough to cause a flow of a sufficient magnitude of the lamp current iL required to turn the lamp on.
In order to obtain a high laser oscillation efficiency, it is necessary for this kind of solid-state laser equipment that the light conversion efficiency from the lamp optical energy to laser output should be high at the laser oscillation unit or that the power conversion efficiency (power factor) of supply power from the AC power supply to input power should be high at the power supply apparatus for laser.
To obtain such a high power conversion efficiency, the above-mentioned conventional power supply apparatus for laser provides a three-phase AC power supply as the input and employs the three-phase rectifying circuit 200 for rectification. Since the three-phase rectifying circuit 200 places across its output terminals a DC line voltage with small ripples, the capacitor 206 is supplied with a charging current ic having few higher harmonics components, to obtain a capacitor charging voltage which is stable against fluctuations in the supply voltage. Thus, power can be supplied to the excitation lamp at a high power conversion efficiency (power factor).
However, the three-phase power supply apparatuses for laser are large, heavy, and expensive and suffers from a big constraint that they can be used only where a three-phase power supply is provided or wired.
In addition, the conventional power supply apparatus for laser has in its main circuit the magnetic contactor 202 comprising electromagnetic relays, so that in emergency of an excessive current etc., the magnetic contactor 202 can be opened (broken) by the control unit 220. The magnetic contactor 202, however, is of such a mechanism that operates mechanical contacts for closing and opening operations (switching). Therefore, this contactor 202 takes a considerable time to complete switching operation after it has received a control signal. With this, it is difficult for this type of mechanism to rapidly break the main circuit if an abnormality such as an excessive current is detected, being insufficient in security.
Also, to obtain a continuously oscillating laser beam by lighting the excitation lamp uninterruptedly, the above-mentioned conventional power supply apparatus for laser must provide switching control over the discharging switching element 208 at a frequency (50 kHz) extraordinarily higher than the power frequency, to supply a continuous (uninterrupted) lamp current i.sub.L to the excitation lamp. Therefore, the switching element 208 is damaged with switching operations and easily destroyed.
If the switching element 208 is destroyed and short-circuited, the excitation lamp current iLbecomes excessive, causing lamp failures. To guard against this, as mentioned above, the current sensor 216 and the current detecting circuit 218 are provided to detect such an excessive current so that the control unit 220 can turn off the magnetic contactor 202 of the main circuit.
However, the destruction of the switching element 208 results in the interruption of the laser oscillation at that point of time. In case of the laser beam processing apparatus, for example, the laser beam processing must be interrupted or stopped at that time, which is disadvantageous in view of the production management or quality control. It is therefore desired to extend the service life of the switching element 208 as long as possible to lessen the frequency at which the laser oscillation is unwillingly interrupted.
As a solution to the above problem, a method is conceivable in which a plurality of switching elements 208 are connected in parallel so that they can perform parallel actions, that is, can be turned on/off at the same time, to thereby reduce the burdens or loads per element.
Typically, however, FETs (Field Effect Transistors), IGBTs (Insulated Gate Bipolar Transistors) , etc., used as such switching elements may often suffer from some unevenness in characteristics (in particular, on-resistance value) among elements. For this reason, in the above method, a larger amount of current may flow concentratedly through a switching element having the lowest on-resistance value among the plurality of switching elements 208 performing the parallel actions. As a result, that switching element may fatigue and destroyed earlier, inconveniently preventing the parallel operation from effective functioning.
In order to start the lighting of the excitation lamp in the conventional power supply apparatus for laser described above, the trigger circuit is operated while the main power supply unit is activated or stand-by. In this case, the excitation lamp is put in high-impedance state up until immediately before the lighting. For this reason, the main power supply unit in the stand-by state applies to the excitation lamp a maximum output voltage equal to the voltage (280V) at the capacitor 206.
Thus, when the impedance of the excitation lamp is lowered by the trigger circuit and the booster circuit 226, an excessive lamp current i.sub.L will flow into the excitation lamp although momentarily, because the lamp voltage from the main power supply unit has a voltage value (280V) which is considerably higher than the rated value (about 150V). This often caused the failures of the lamp.
Furthermore, in the conventional power supply apparatus for laser as described above, the AC power supply voltage e is stepped up by the step-up transformer 224, and the secondary voltage of the transformer 224 is further boosted by the step-up circuit or the booster circuit 226, to thereby generate a boost voltage E.sub.f having a desired voltage value.
However, such a boosting method may suffer from a deficiency that it is susceptible to fluctuation of the AC power supply voltage, resulting in an unstable boost voltage E.sub.f. More specifically, if the AC power supply voltage e fluctuates, the amount of the fluctuation may be boosted up to several times (e.g., ten times) by the step-up transformer 224 and the booster circuit 226, which reflects on the boost voltage E.sub.f.
As a result, the excitation lamp may not light up due to the fact that the boost voltage E.sub.f is considerably lower than the preselected voltage value (2500V) at the start of the lighting of the excitation lamp. On the contrary, the excitation lamp may break down due to an extremely high boost voltage E.sub.f.